口吃 (kǒu chī): à stutter

He has had a stammer since childhood; when he gets nervous, he stutters and often has to repeat several words before he can finish a sentence.

To overcome her stammer, she practices pronunciation in front of the mirror every day and has joined a public speaking club; now she speaks much more fluently.

When the teacher called on him to answer in class, he got so anxious about his stammer that his face turned red; his classmates all patiently waited for him to finish speaking slowly.

Although that actor has a slight stammer, once he steps on stage he becomes a completely different person, delivering his lines clearly and powerfully.

The doctor told him that stuttering can be improved through speech therapy and encouraged him not to fear communicating with others just because his speech isn't fluent.